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The Evolution of Prebiotic Chemical Complexity

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The Evolution of Prebiotic Chemical Complexity National Aeronautics and Space Administration NASA Astrobiology Institute Annual Science Report 2016 Team Report: The Evolution of Prebiotic Chemical Complexity and the Organic Inventory of Protoplanetary Disk and Primordial Planets, NASA Ames Research Center The Evolution of Prebiotic Chemical Complexity and the Organic Inventory of Protoplanetary Disk and Primordial Planets Lead Institution: NASA Ames Research Center Team Overview The Evolution of Prebiotic Chemical Complexity and the Organic Inventory of Protoplanetary Disks and Primordial Planets Team seeks a greater understanding of the chemical processes that occur at every stage in the evolution of organic chemical complexity, from quiescent regions of dense molecular clouds, through all stages of disk and planet formation, and ultimately to the materials that rain down on planets. The effort is an integrated, coherent program involving the interaction of a number of well-integrated research projects: • Modeling and observations of protoplanetary disks • Modeling and observations of exoplanets • Laboratory studies of gas-grain chemistry • Laboratory studies of ice photolysis • Computational quantum chemistry Principal Investigator: Scott Sandford These projects interact closely with each other so that each benefits from advances made in the others and helps guide future work. For example, the modeling of the chemistry that takes place in protostellar disks benefits from inputs provided by spectral, physical, and chemical properties of molecules determined by the laboratory and computational projects, but also provides guidance for key areas of future computational and laboratory work. Similarly, the computational studies can be used to help interpret laboratory results and extend them to additional materials or environments, while the lab results can provide confirmation of computational reaction paths. Team Website: http://amesteam.arc.nasa.gov NASA Astrobiology Institute 1 Annual Report 2016 2016 Executive Summary During 2016 our Team made significant progress on all aspects of our combined research. Some highlights are described briefly below. More details can be found in our individual project reports. Disk Modeling - We have developed global protoplanetary disk evolution models that incorporate the formation of planetesimals in a 1+1D viscous evolution framework that combines the effects of photoevaporation, dust evolution, turbulence, and planetesimal formation by the streaming instability. This work shows that, as the gas is removed, regions with an over-density of dust grains are created that lead to the formation of planetesimals on relatively short timescales, i.e., planetesimal formation in the outer disk is a natural outcome of disk evolution. We have also begun incorporating gas-grain chemistry into the disk-modeling network to study transport of ices within disks. Exoplanets - Our Team is at the leading edge of extrasolar planet discovery, and we are core participants in the long-running Lick-Carnegie Exoplanet Survey. Over the past year we assembled and published a compendium of 60,949 precision radial velocities for 1,624 nearby sun-like stars and red dwarfs. This data, all taken with the Keck-I Telescope over a period of more than 20 years, supports the detection of hundreds of exoplanets, including 60 new candidate worlds. Our planet detection efforts also extend to our own Solar System. Strong dynamical evidence points to the potential existence of “Planet Nine”, a ~10 Earth-mass world on an eccentric ~17,000-year orbit. Our work pinpoints the as-yet unseen object at a distance of 950 AU, with a sky position near RA=3 hours, Dec=0 degrees, and apparent brightness V~23. Laboratory Studies - Significant progress has been made in several areas of our laboratory research. An apparatus for studying gas-grain chemistry has been developed and constructed. We have also completed a preliminary study of the abiotic production of sugars and sugar derivatives during the photolysis of simple ice mixtures. We have also completed studies that have demonstrated that all of the nucleobases used by modern life, as well as a host of related compounds, can also be produced by ice photolysis (Fig. 1). Sample TIC Mass = 279 Da Adenine Standard 12.6 12.65 12.7 12.75 12.8 12.85 12.9 Time (min) Fig. 1. Left panel: Total-ion chromatogram (TIC) of the residue produced from a UV-irradiated H2O:NH3:purine ice (top), single-ion chromatogram (SIC) of the same residue for mass 279 Da (middle), and SIC of the adenine standard (mass 279 Da). Right panel: Mass spectrum of the peak identified as adenine in the residue produced from a UV-irradiated H2O:NH3:purine ice (top), an adenine standard (middle), and 15 shifted mass spectrum of adenine in a residue produced from an irradiated ice containing NH3. NASA Astrobiology Institute 2 NASA AmesAnnual Research Report Center 2016 Computational Quantum Chemistry - In conjunction with the laboratory studies of nucleobase production, we investigated the formation of nucleobases in irradiated astrophysical ices containing pyrimidine and purine using computational quantum chemistry. This work demonstrated that the formation of nucleobases is energetically and kinetically favorable given the presence of one or several water molecules, i.e., it can occur in the solid state, but not in the gas phase (Fig. 2 and 3). This work also explains why some nucleobases (uracil) are made efficiently, while others (thymine) are not. In another study, electronic structure calculations were performed on + + + + C4H3 , C6H3 and C6H5 . C6H5 was found to be very stable and may be a good nucleation center for Fig. 2. Diagram illustrating the three reaction pathways from pyrimidine 1 to the growth of larger polycyclic uracil 3 and thymine 6 we studied using quantum chemical computations. aromatic hydrocarbons (PAHs), one of the most abundant families of molecules in space. We also continued our work with the Dutch Astrochemistry Network (DAN) on the computation of infrared emission from PAHs. Team Members Partha Bera Uma Gorti Martin Head-Gordon Gregory Laughlin Timothy Lee Andrew Mattioda Michel Nuevo Xander Tielens Fig. 3. Reaction scheme indicating the amino and hydroxyl group substitutions on the purine molecule and pathways to the nucleobases adenine and guanine Gustavo Cruz-Diaz studied with quantum chemical computations. Christopher Materese Tamar Stein NASA Astrobiology Institute 3 Annual Report 2016 Project Reports Planetesimal Formation and Transport of Ices We have begun incorporating gas grain chemistry in Protoplanetary Disks into the disk modeling network to study transport We developed global protoplanetary disk of ices. To that end, a gas-grain chemical network evolution models that incorporate the formation of was added to augment our current gas-phase planetesimals in a 1+1D viscous evolution framework. and photo-reaction chemistry in disk models. We combined the effects of photoevaporation, dust Dust grains continually collide and fragment or evolution, turbulence, and planetesimal formation coagulate (depending on local conditions in the by the streaming instability to study how dust disk) throughout the disk as it evolves, leading to forms planetesimals in an evolving disk. As the gas their size redistribution. Molecules preferentially is removed by photoevaporation, dust is retained freeze on small grains, which have greater total in the disk since flow gas densities are too low to cross-sectional area. Ices moreover change the local entrain dust. A steady increase in the amount of dust fragmentation threshold for destructive collisions relative to gas triggers the streaming instability due compared to bare grains, as ices are more “sticky” and to the back-reaction of the dust with the gas. Regions facilitate coagulation. We have constructed simple with an over-density of dust grains are created and semi-analytical formulations to include the effects lead to the formation of planetesimals on relatively of gas-grain chemistry and freeze-out to study the short timescales. Our models produce ~76-153 distribution of CO and H2O ice as the disk evolves. Earth masses of planetesimals. Thus, planetesimal Parameter studies involving a few species at a time formation in the outer disk is found to be a natural are being conducted to gain a better understanding outcome of disk evolution. of ice chemistry in disks. Fig. 4. The total mass in solids in different size ranges (color-coded) evolves with time as the disk disperses, and is shown in the figure above. The streaming instability can efficiently convert dust (<1 cm) into planetesimals (in blue), and in the model here, about 76 Earth masses of dust is converted during the disk lifetime of about 3 million years. NASAThe Evolution Astrobiology of Prebiotic Institute Chemical Complexity and the Organic 4 NASA Ames Research Center Inventory of Protoplanetary Disks and Primordial Planets Exoplanet Studies We have continued our radial velocity surveys to In order to aid its detection, and to assess its likely detect and characterize extrasolar planets. The properties, we constructed atmospheric models that highlight of the past year’s effort has been the predict its reflection and emission spectra in the publication of the full catalog of high-precision optical and the infrared. We found that it should have velocities obtained over the past two decades with a very pure H-He atmosphere in which methane and the Keck Telescope (Butler, Vogt, Laughlin et al. other volatiles are fully condensed into deep-layer 2017). This compendium of 60,949 precision Doppler cloud. It should thus have a high albedo (and a light- measurements is accompanied by detailed spectral blue overall appearance) characterized by Rayleigh indices, tabulated host star properties for 1,624 scattering. (Fortney, Marley, Laughlin et al. 2016). nearby sun-like stars and red dwarfs, and a catalog of Through a Monte-Carlo survey, we established that sixty new candidate exoplanets, including a low-mass if it exists, Planet Nine is very likely (P~98%) in low- world orbiting Lalande 21185, the fourth nearest order mean-motion resonances with the most distant main-sequence stellar system to the Sun.
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